In the United States, electrical outlets have become standardized to older, two-prong and newer, three-prong configurations. Currently, the three-prong configurations include a narrow opening or slot for the voltage prong of an electrical plug; and the neutral slot is somewhat wider. The third opening is for a ground connection; and, for three-prong plugs, this ground connection ensures the proper orientation of the plug in the outlet.
Typically, the exposed face of standard outlets has narrow openings which prevent even small children""s fingers from coming into contact with any dangerous electrical current located within the receptacle. Children,-however, sometimes insert small metal objects, such as bobby pins, paper clips and screwdrivers, into the narrow openings of electrical outlets, with occasionally disastrous results.
Standard electrical outlets of the type described above also have an inherent hazard in conjunction with partially inserted plugs. The prongs of the plugs typically make electrical contact with the voltage and neutral lines through internal contacts in the plug before the plug is fully inserted. The small fingers of children, and even the fingers of adults, can bridge across the prongs of a partially inserted plug, resulting in an electrical shock and/or burn. If the child or adult has another part of his body in contact with an electrical ground, a serious injury or even fatality can result from such a shorting across the prongs of a partially inserted plug.
Attempts have been made in the past to provide safety features in electrical outlets to prevent electrical shock and injury. For example, the device of U.S. Pat. No. 2,540,496 to Sperrazza, is directed to an electrical outlet having a non-conducting cam block located approximately half-way down the length of each plug prong slot. This cam block forces together the contact points for the opposite side of the receptacle when plug prongs are inserted approximately half-way into the electrical outlet. Consequently, the outlet is made electrically hot while at least one half of the prongs of the plug being inserted are still exposed to contact by the person inserting the plug. This is required to prevent electrical arcing in the electrical outlet.
U.S. Pat. No. 2,751,527 to Bissell discloses an explosion-proof electrical connector system requiring a specialized electrical outlet and specialized mating electrical plug. There is no teaching of how to utilize this invention with a conventional residential or commercial electrical outlet commonly used for lamps, small appliances, office equipment and small power tools.
A different approach to providing safety measures in a common electric duplex outlet is disclosed in the U.S. Pat. No. 3,669,285 to Leatherman. This patent shows a device using a specialized form of the ground prong on a three-prong plug to make the electrical outlet hot. The device of this patent will not work with common polarized two-prong plugs, and only works with a three-prong plug having special configurations.
Yet another approach to using the ground prong of a three-prong plug to activate an electrical outlet is disclosed in the U.S. Pat. No. 3,755,635 to McGill. This patent discloses use of the ground prong to activate a low current microswitch, which in turn activates a relay allowing electrical current to flow to the plug. While not disclosed, the relay apparently is large enough to cause a standard duplex outlet to be able to contain only one plug outlet. There is no teaching of how to use a polarized two-prong plug (without ground) with this invention. In fact, such a plug, common on most small appliances, will not work with the device disclosed in this patent.
A safety duplex outlet for polarized two-prong and three-prong plugs is disclosed in the U.S. Pat. No. 5,095,182 to Thompson. The device described in this patent teaches the use of a microswitch mounted behind the large (neutral) slot of a standard polarized wall outlet. A person inserting a wire, paper clip or the like of any width which fits into the neutral slot of the outlet, could activate the microswitch at the bottom of the slot, and thereby circumvent the safety feature of the device of this patent. The addition of the microswitch mounted on the bottom of the outlet also prevents mounting of the outlet in a standard electrical box.
U.S. Pat. No. 5,347,095 to Zeder discloses an electrical outlet which emits an audible warning when an electrical plug is removed from it. There is no teaching of how to provide an electrically safe outlet, however.
U.S. Pat. No. 5,513,999 to Fry discloses activation of an outlet/plug assembly upon partial insertion of the plug into the outlet. The outlet and the plug, however, are of non-standard designs, which requires the replacement of both the common standard duplex outlet and the commonly provided polarized two-prong and three-prong plugs in order to use this outlet/plug assembly.
An improvement over all of the foregoing patents is disclosed in the device of U.S. Pat. No. 6,111,210 to Allison. This patent discloses an electrical safety outlet which activates only the outlet into which a plug is inserted in a multi-outlet receptacle; so that the unused outlets are not made electrically hot. The device of the Allison patent employs internal voltage contacts in the voltage slot for engagement by the voltage prong of a plug. The voltage line contact is not connected directly to the internal voltage contact, but instead is connected to one side of a switch, the other side of which is connected to the internal voltage contact. A cam is located in the neutral slot of the receptacle for engagement by the neutral prong of a plug inserted into the receptacle. When the prong reaches near the bottom of the neutral slot, the cooperation of a pair of cams functions to close the switch to apply voltage to the internal voltage contact. A single switch is used to interconnect the line voltage with the internal voltage contact to activate the plug. The placement of the cams is selected so that operation of the switch is extremely difficult unless the blade or prong of a standard electrical plug is inserted into the neutral side of the outlet.
The Allison patent discloses various cams and operating levers for effecting the closure of the voltage switch; and operation of both of the cams, or of the other mechanisms, with a paper clip, hairpin or the like, is nearly impossible. Even so, since the closure of electrical current to the voltage side of the receptacle is effected through operation of the neutral side only, it is remotely possible for the plug to be activated with an open voltage slot exposed.
It is desirable to provide an improved shock-resistant electrical outlet which is simple to construct and operate, and which provides an increased level of safety through the use of multiple series-connected switches between the line voltage contact and an internal voltage contact.
It is an object of this invention to provide an improved shock-resistant electrical outlet.
It is another object of this invention to provide an improved shock-resistant electrical outlet providing enhanced safety features with standard electrical plugs.
It is an additional object of this invention to provide an improved shock-resistant electrical outlet utilizing multiple series connected switches actuated by the prongs of an electrical plug and connected in series between a voltage line contact and an internal voltage contact in the outlet.
It is a further object of this invention to provide an improved electrical outlet in which no connection is made between a voltage line contact and an internal voltage contact engaged by the voltage prong of a plug until both the neutral prong of a plug and the voltage prong of a plug are nearly fully inserted into the outlet, to each engage actuators, all of which must be moved to interconnect series-connected switches between the voltage line contact and the internal voltage contact to energize the outlet.
In accordance with a preferred embodiment of the invention, a shock-resistant electrical outlet is constructed in the form of a plug receptacle having at least two slots, one of which is a voltage slot for receiving the prongs of an electrical plug. An internal voltage contact is provided in the voltage slot for engagement by the voltage prong of a plug; and an internal contact is provided in the other slot for engagement by the other prong of a plug. A line contact is conductively connected to the internal contact in the other slot. A voltage line contact is connected in series with at least two normally open switches connected in series electrical circuit between the voltage line contact and the internal voltage contact. Switch actuators are located for simultaneous engagement by both prongs of a plug inserted into the receptacle to close the normally open switches to complete the electrical circuit between the voltage line contact and the internal voltage contact of the receptacle.